Air pollution by petroleum products is one of the most urgent problems of our present society. Freedom of travel in daily activities is directly affected by the availability of gasoline for motor vehicles. For example, approximately 2.2 billion gallons per year of motor vehicle gasoline are consumed within the San Francisco Bay area air pollution control district. Marketing of such gasoline involves the transfer of gasoline from one container to another, as for example, from a refinery storage tank to a bulk handling motor vehicle tank truck, thence from the tank truck to an underground storage tank at a service station, and thence from the underground storage tank to an automobile gasoline tank. The transfer of liquid gasoline from one container to another container produces gasoline rich vapors which are displaced into the atmosphere as the container is filled. In the Bay Area district it is estimated that 75 tons per day of gasoline enter the district area atmosphere.
Air pollution in the form of haze and smog formation includes breakdown of hydrocarbons of a type found in gasoline vapor. Gasoline marketing operations involving transfer of gasoline from one container to another container require careful consideration if improvements in air quality are to be achieved.
The present invention is primarily concerned with, but not limited to, reducing gasoline vapor losses at a service station area which arise during the transfer of gasoline from a bulk tank truck to an underground storage tank and thence from the underground storage tank through the gasoline pumps to an automobile tank. Gasoline vapor losses at the service station principally arise from the underground storage tank which is subjected to both breathing and displacement losses. Breathing losses are caused by alternate expansion and contraction of the tank contents due to day-night temperature differentials. Such temperature differentials are minimized by using buried tanks at gasoline storage stations. Displacement losses occur upon refilling a partially empty or empty storage tank which normally expels an equivalent volume of vapor into the atmosphere through the vent pipe of the storage tank. While there may be spilling losses in the area of the service station, such losses are relatively insignificant compared to the losses caused by breathing and displacement.
While it is readily understandable that gasoline vapor emissions may be produced by breathing and displacement conditions at a storage tank, a solution to the vapor emission control problem requires attention to boiling range of gasoline being handled. Boiling range establishes the volatility that the gasoline liquid must have in order to be effectively utilized in an internal combustion engine. In a refinery operation, crude oil is processed and its components so blended and ultimately pressurized that the finished gasoline product has a final desired vapor pressure designated "Reid Vapor Pressure" or RVP. In addition, the finished gasoline product comprises components, the heaviest of which, will readily vaporize in the gasoline engine.
For example, gasoline having a RVP of about 7.5 at 60.degree. F. will produce vapors having about a 50/50 mixture of hydrocarbon and air. If such vapors are continuously replaced with fresh air it is possible to vaporize a very large amount of liquid gasoline. In an automobile fuel tank, air replaces the volume of fuel consumed during driving. This tank air volume will be the sole source of air to replace liquid and vapors in the preceding storage container from which liquid gasoline was drawn to fill the automobile tank, that is, the storage tank at the gas station. At the service station, vapors from the automobile fuel tank could ultimately be transferred through the storage tank to the emptied gasoline truck for return to the refinery depot.
It should also be noted that if an automobile tank is refueled directly from a delivery truck tank which is normally vapor tight, the delivery tank will obtain its displacement vapor only from the vapor space of the automobile tank as the fuel is dispensed. Thus, from the automobile tank to the delivery tank, liquid is being exchanged for gasoline saturated vapor volume. If the two tanks are at the same temperature, then the exchange of volume will be on a one to one basis. But if the delivery tank temperature is higher and colder tank displaced vapors come to equilibrium temperature, then all of the vapor from the automobile tank will not fit, in expanded condition, into the delivery tank and excess vapor will escape into the atmosphere as a vapor loss. If the delivery tank is cooler, then the vapors transferred to the delivery tank will contract and outside air must be sucked into the vent line of the delivery tank, or gas vaporized or the tank pressure remains below atmospheric pressure to make up the difference in volume.
Prior to vapor control systems and where an automobile fuel tank had only one or two gallons of gasoline remaining, this small amount of gasoline was considered to be highly "weathered" because of engine heat, high agitation and vehicle tank ventilation. By weathered is meant that the gasoline has lost some of its more volatile components. Vapor space in the automobile tank is saturated with respect to volatile components and their mole fractions in the liquid and vapors. When the automobile tank is filled with fresh gasoline more gasoline vapors are produced as gasoline is used reflecting the changed composition of the fresh gasoline. Volume of vapors discharged from the vehicle tank during refueling may be from 2 to 15 percent greater than the liquid volume of the gasoline dispensed. Various prior proposed systems have been used to cope with this problem including vapor balanced transfer systems where liquid and vapor spaces are connected together between the two containers in which liquid is to be transferred, absorption with lean oil, high pressure compression systems, adsorption of hydrocarbon vapors on activated charcoal, refrigeration of saturated vent gas, compression and refrigeration of the vent gases, and combustion devices to dispose of residual hydrocarbons in vented gases. Extremely elaborate vapor recovery systems for service station installations do not appear to be economically justified because of the small net volume of hydrocarbons to be recovered.
The present invention is directed toward a system in which the vented gases are burned and combustion efficiency is maintained even though there is a wide range in the variable characteristics of the vented gases.